[Rd] sprintf, check number of parameters

Duncan Murdoch murdoch@dunc@n @end|ng |rom gm@||@com
Mon Feb 22 11:34:26 CET 2021

This is ugly, but I think it's legal, and it doesn't trigger a warning: 
  output unused parameters as zero-length strings:

  msnx(T0, mask = '%1$.1f (SD=%2$.1f)%3$.0s%4$.0s')

Perhaps an example using %.0s could be included to show how to skip a value.

Duncan Murdoch

On 22/02/2021 5:06 a.m., Tomas Kalibera wrote:
> Dear Matthias,
> On 2/6/21 2:11 PM, Matthias Gondan wrote:
>> Dear developers,
>> This is a follow-up from an earlier mail about warnings of unused arguments in sprintf:
>> 1. This should obviously raise an error (and it does):
>> sprintf('%i %i', 1)
>> Fehler in sprintf("%i %i", 1) : zu wenig Argumente [= too few arguments]
>> 2. This should, in my opinion, raise a warning about an unused argument (and I think it does in now R-devel):
>> sprintf('%i', 1, 2)
> yes, it does.
>> 3. From the conversation below, it seems that this also raises a warning (in R-devel):
>> sprintf('%1$i', 1, 2)
> yes, it does as well
>> I think that one should be suppressed. When I reported this a few months ago, I didn’t really have a use case for (3), but now I think I have found something. Suppose I have a function that calculates some descriptive statistics, mean, sd, available cases, missings, something like the one below:
>> msnx = function(x, mask='%1$.1f (SD=%2$.1f, n=%3$i, NA=%4$i)')
>> {
>>     m = mean(x, na.rm=TRUE)
>>     s = sd(x, na.rm=TRUE)
>>     n = sum(!is.na(x))
>>     na = sum(is.na(x))
>>     sprintf(mask, m, s, n, na)
>> }
>> The mask is meant to help formatting it a bit.
>> msnx(T0)
>> [1] "30.7 (SD=4.7, n=104, NA=0)"
>> Now I want a „less detailed“ summary, so I invoke the function with something like
>> msnx(T0, mask='%1$.1f (SD=%2$.1f)')
>> [1] "30.7 (SD=4.7)"
>> In my opinion, in the last example, sprintf should not raise the warning in (2) if all arguments in the mask are „dollared“. I am still a bit unsure since the example uses a function that calculate things that aren’t being used (n and na), and this could be considered bad programming style. But there might be other use cases, and it is, nevertheless, a deliberate choice to skip arguments 3$ and 4$.
> Thanks for the example. I am sympathetic with your concerns about the
> programming style in it: the caller needs to know exactly how "mask"
> will be used, that it would be in a call to sprintf() and what would be
> the indices of the arguments.
> The warning has been introduced a while ago and there has not been any
> report yet that it would break existing good style code (particularly
> CRAN packages have been tested extensively), which indicates that
> currently the R code base does not rely on unused $- arguments.
> It is hence I think wise to keep the warning to prevent R code base from
> relying on that in the future, because gcc/clang already warn on unused
> $-arguments. Not only that gcc developers must have been thinking hard
> about the same thing before us getting to this conclusion: $- arguments
> are a POSIX extension and gcc/clang are the key compilers for POSIX
> systems, so it is safer to abide by their rules. In principle POSIX may
> mandate that $- arguments are used explicitly in the future (now it is
> rather vague, it seems unused are fine only when last), and even if not,
> deviations from gcc/clang could cause confusion for applications and
> developers using both C/C++ and R.
> Best
> Tomas
>> Best wishes,
>> Matthias
>> Dear Matthias,
>> thanks for the suggestion, R-devel now warns on unused arguments by
>> format (both numbered and un-numbered). It seems that the new warning is
>> useful, often it finds cases when arguments were accidentally passed to
>> sprintf but had been meant for a different function.
>> R allows combining both numbered and un-numbered references in a single
>> format, even though it may be better to avoid and POSIX does not allow
>> that.
>> Best
>> Tomas
>> On 9/20/20 1:03 PM, Matthias Gondan wrote:
>>> Dear R developers,
>>> I am wondering if this should raise an error or a warning.
>>>> sprintf('%.f, %.f', 1, 2, 3)
>>> [1] "1, 2"
>>> I am aware that R has „numbered“ sprintf arguments (sprintf('%1$.f', …), and in that case, omissing of specific arguments may be intended. But in the usual syntax, omission of an argument is probably a mistake.
>>> Thank you for your consideration.
>>> Best wishes,
>>> Matthias
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